The present invention relates to an optical paper sorter, and in particular to a device and method for determining if a piece of paper is white or non-white.
In the paper recycling business, different grades of paper typically have different values, and thus there is a need to sort incoming recyclable paper products into various grades. Generally, the value of white paper exceeds the value of paper that is not white, and accordingly it is common to separate white recyclable paper from non-white recyclable paper. In the past, such sorting has been done manually, which tends to be expensive and has a varying degree of accuracy.
In other industries, the use of diffuse reflectance analysis has been applied to assist in sorting various types of work pieces, based on colour. For example, U.S. Pat. No. 4,278,538 issued Jul. 14, 1981 to Lawrence et al discloses a sorting system for sorting telephone caps of uniform colour in which diffuse reflection from the caps is analyzed to determine the colour of the telephone caps. However, despite the use of diffuse reflectance analysis in other industries, it has not been adopted in the paper sorting industry. A unique problem faced in determining if a sheet of paper is white or non-white is that recyclable material, by its nature, generally includes printed or graphic information on its surface. Accordingly, in order to successfully distinguish between non-white and white paper products, an automated sorting system must be able to, with reasonable accuracy, distinguish white paper having printed and graphics material on its surface from non-white paper (which may also include white elements).
Accordingly, it is desirable to provide a device and method for determining the dominant colour of a piece of paper, and more particularly for determining whether a piece of paper can be categorized as white or non-white. It is also desirable to provide a device for redirecting pieces of paper depending on if they are white or non-white.
The present invention provides an optical sorter that measures the diffuse reflectance of an incident light beam on a piece of paper or other workpiece, and processes the measured values to catagorize the piece of paper or other workpiece as falling within one of two possible colour classifications.
According to one aspect of the invention, there is provided a device for determining the dominant colour of a workpiece. The device includes a light source for directing a beam of light at the workpiece to illuminate the workpiece, and an optical detection system for receiving light diffusely reflected off the workpiece, isolating three different spectral components of the reflected light, measuring the intensity of each of the three different spectral components and generating electrical signals representative of the intensity of each of the three different spectral components. A processor responsive to the electrical signals generated by the detection system is operable to determine a relative reflectance for each of the three spectral components, determine a mean of the three relative reflectances, determine a standard deviation of the three relative reflectances, and determine, by comparing the mean and standard deviation to predetermined threshold values, a probable dominant colour of the workpiece. Preferably, the optical detection system includes three photo detectors for receiving light diffusely reflected off the workpieces, and filters positioned between the photo detectors and the workpiece for isolating the reflected light into the three different spectral components such that each of the three photo detectors receives a different one of the spectral components and generates an electrical output representative of the intensity thereof.
Preferably , the processor is configured to determine the relative reflectance for each of the three spectral components by comparing the intensity of each of the three spectral components to predetermined reference intensity values obtained in respect of a reference workpiece of a known colour classification, and the processor is configured to determine the probable colour of the workpiece by classifying the workpiece as falling into one of two possible colour classifications, one of which is the known colour classification.
According to a further aspect of the invention, there is provided a paper sorting device for determining if the dominant color classification of a piece of paper is white or non-white, comprising a light source for directing a beam of light at the paper to illuminate it, and an optical detection system for receiving light diffusely reflected off the paper, isolating three different spectral components of the reflected light, measuring the intensity of each of the three different spectral components and generating electrical signals representative of the intensity of each of the three different spectral components. A processor responsive to the electrical signals generated by the detection system is operable to determine a relative reflectance for each of the three spectral components, determine a mean of the three relative reflectances, determine a standard deviation of the three relative reflectances, and determine, by comparing the mean and standard deviation to predetermined threshold values, whether the paper is white or non-white. Preferably, the wavelength ranges of the three spectral components are generally 400 nm-525 nm, 475 nm-650 nm, and 600 nm-800 nm, respectively. Conveniently, the device may include a conveyor system for advancing pieces of paper to and through a sampling station at which the light source is located, and an ejection device connected to said processor and being operable to selectively redirect a paper sample from the conveyor system, the processor being configured to cause the ejection device to redirect the paper sample from the conveyor system based on the determination of whether the paper sample is white or non-white.
According to still a further aspect of the invention, there is provided a method for classifying paper samples into one of two colour classifications, comprising the steps of directing a beam of visible light on the paper sample to illuminate a paper sample, measuring the intensity of three different spectral components in the light which is diffusely reflected off the paper sample, determining, based on the measured intensities, a relative reflectance for each of the three spectral components, determining a mean of the three relative reflectances, determining a standard deviation of the three relative reflectances, and comparing the mean and the standard deviation to predetermined values and classifying the paper sample as falling within one of two possible colour classifications based on the comparison results. Preferably, the two colour classifications are white and non-white and the first spectral component corresponds to the colour blue, the second spectral component corresponds to the colour green, and the third spectral component corresponds to the colour red.